The present application is a continuation of International Application No. PCT/SE00/01224, filed Jun. 13, 2000, published Jan. 11, 2001, which claims priority to Swedish Application No. 9902567-8, filed Jul. 5, 1999. Both applications are expressly incorporated herein by reference.
1. Technical Field
The present invention relates to a method for controlling a direct injected combustion engine. More particularly, the invention relates to controlling a direct injected Otto cycle combustion engine two modes of operation wherein the two modes have different air/fuel mixtures supplied to the engine. These operation modes preferably include a first mode for stratified operation and a second mode for homogeneous operation. The invention also relates to a direct injected engine arrangement for such control.
2. Background Information
In vehicles operated by combustion engines, there is a general demand for low emissions of harmful substances in the exhaust gases from the engine. These substances primarily include pollutants in the form of nitrous oxide compounds (xe2x80x9cNOxxe2x80x9d), hydrocarbon compounds (xe2x80x9cHCxe2x80x9d), and carbon monoxide (xe2x80x9cCOxe2x80x9d). For gasoline engines, the exhaust gases are normally purified by an exhaust catalyst that forms part of the exhaust system and through which the exhaust gases flow. In a three-way catalyst known in the art, the major part of the above harmful compounds is eliminated by known catalytic reactions. In order to optimize the function of the catalyst so that it provides an optimal degree of purification for NOx, HC, and CO, the engine is typically operated by a stoichiometric air/fuel mixture, i.e., a mixture where xcex=1.
Furthermore, there is a general demand for reducing vehicle fuel consumption by the engine to the greatest extent possible. To this end, engines have been developed during recent years with new types of combustion chambers in the engine cylinders, in particular so that the engine can operate with increasingly lean fuel mixtures, i.e., where xcexxe2x89xa71. In a DI engine (ie., a direct injected Otto cycle engine), each cylinder combustion chamber in the engine is constructed so that the fuel supplied is highly concentrated at each respective ignition plug. This operating mode is generally termed xe2x80x9cstratifiedxe2x80x9d operation. During continuous driving at low or medium-high torque and engine speed, stratified operation provides an operation with a very lean air/fuel mixture, e.g., an operating mode of up to about xcex=3. In this manner, a substantial reduction in fuel consumption is obtained. The engine can also be operated in an xe2x80x9chomogeneousxe2x80x9d mode of operation with an essentially stoichiometric mixture (xcex=1) or a comparatively rich mixture (xcex less than 1). This later mode of operation normally prevails during driving situations with comparatively high torques and engine speeds.
During stratified operation, a lean exhaust gas mixture flows through the three-way catalyst. In doing so, the three-way catalyst becomes saturated so that it can not be utilized for reducing NOx compounds in the exhaust gases. This is due to the fact that it is constructed for an optimal degree of purification for a stoichiometric mixture. For this reason, a conventional three-way catalyst can be combined with a nitrous oxide adsorbent, or NOx adsorbent or trap, This adsorbent or trap is known per se for adsorbing NOx compounds, eg., in the exhaust gases from a combustion engine. In this manner, the NOx adsorbent can be utilized to compliment a conventional three-way catalyst. This can be done either as a separate unit upstream of the three-way catalyst or as an integral part of the three-way catalyst, i.e., together with the catalytic material of the three-way catalyst. In the latter, an integrated component in the form of a NOx adsorbing exhaust catalyst is formed.
The NOx adsorbent is constructed so that it takes up (adsorbs) NOx compounds in the exhaust gases when the engine is operated by a lean air/fuel mixture and gives off (desorbs) the NOx compounds when the engine is operated by a rich air/fuel mixture during a certain time period. Furthermore, the NOx adsorbent has the characteristic of being able to adsorb NOx compounds only up to a certain limit, i.e., it is eventually xe2x80x9cfilledxe2x80x9d, reaching an adsorption limit. In this situation, the NOx adsorbent must be regenerated, i.e. it must be desorb and release the accumulated NOx compounds. If a conventional three-way catalyst is arranged downstream of a NOx adsorbent, or if the three-way catalyst is formed as an integral part of a NOx adsorbent, the desorbed NOx compounds can be eliminated by means of the three-way catalyst, provided that the catalyst has reached its ignition temperature.
A NOx adsorbent can be regenerated by providing a comparatively rich exhaust gas mixture flow through the NOx adsorbent becomes during a certain time period of approximately a few seconds. This is accomplished by operating the engine in the homogeneous operating mode during this time period, wherein the engine runs with a comparatively rich air/fuel mixture. By doing so, the NOx adsorbent is xe2x80x9cemptiedxe2x80x9d so that it can subsequently adsorb NOx compounds for a time frame that lasts until the adsorbent is saturated and a new regeneration is needed.
It is known to switch between stratified and homogeneous mode by adjusting the air/fuel mixture supplied to the engine and the length of time for injecting the mixture. This switch is normally initiated due to the vehicle driver requesting a change in torque from the engine. This request can be provided by detecting the position of the engine""s accelerator pedal. The vehicle includes a computer based control unit that, depending on the required torque, sees that a suitable air/fuel mixture is fed to the engine depending on, for example, whether a stratified or a homogeneous operation is required, or the required torque and engine speed of the engine.
In addition to a driver-initiated switch from an operating mode such as from stratified to homogeneous operation, the engine must also be able to switch to the homogeneous mode of operation in a compulsory manner (i.e., regardless whether the prevailing driving situation corresponds to homogeneous operation or not) when the control unit has estimated or determined that the NOx adsorbent needs to be regenerated. This requirement typically occurs after a certain time period has passed from a previous regeneration, or after a certain amount of NOx compounds has been fed into the NOx adsorbent, filling when the NOx adsorbent. A control unit for providing this type of operation is provided with a suitable strategy for switching the combustion engine between homogeneous and stratified operation based on the amount or degree of throttle application and engine speed, and considering whether NOx regeneration is necessary.
However, in previously known systems for switching a direct injected engine between stratified and homogeneous operation, a problem can occur when this mode switch happens while operating the engine with an air/fuel mixture composition that is disadvantageous for the required operating case. This is because each operating case of the engine requires a certain optimized air/fuel mixture. For example, a switch to a rich, stratified mode of operation can result in undesired carbon deposits in the engine. Furthermore, a switch to a lean, homogeneous operating mode results in an unstable combustion. This, in turn, can result in loss of torque or misfire of the engine.
The problem regarding disadvantageous air/fuel mixtures (i.e., disadvantageous lambda values) for each operating mode can per se be solved by adjusting the fuel amounts to the engine, thereby achieving a suitable lambda value. However, such a procedure can negatively impact the torque of the engine. Obviously, such an impact can affect the comfort or drivability of the vehicle, causing a xe2x80x9cjerkxe2x80x9d for the passengers therein.
Accordingly, there is a need for a method of controlling a direct injection combustion engine that provides for a switch in operation between stratified and homogeneous modes without negatively affecting the drivability of the vehicle or engine torque. This method should further provide an optimized air/fuel mixture that minimizes negative exhaust emissions.
The present invention provides an improved method for controlling a combustion engine. In particular, the invention provides an optimized switch between at least two operating modes in a direct injection engine. Preferably, the modes are the homogeneous and stratified operating modes. The method permits the mode switching to occur while adjusting the air/fuel mixture so that correct operation of the engine happens in both modes of operation during the switch. The present invention also provides an arrangement that includes a generator for generating an air/fuel mixture to the engine cylinder(s), a means or control unit for switching between at least two modes of operation by adjusting the air/fuel mixture supplied to the engine and the amount of time for injection the mixture. The arrangement is such that the operating switch is initiated when the engine is provided with an air/fuel mixture adjusted so that correct operation of the engine occurs in both operating modes during the switch.
The invention relates to a method for controlling a direct injected, or DI combustion engine. The method includes generating an air/fuel mixture to the respective cylinders of the engine, and switching between at least a first operating mode and a second operating mode of the engine by adjusting the air/fuel mixture supplied to the engine and the amount or length of time for injecting the mixture. The method further includes initiating the operating mode switch in connection with operating the engine with an air/fuel mixture that is adjusted so that correct operation of the engine is allowed in both operating modes during the switch.
By using the invention, several advantages are accomplished. Primarily, the time for switching between the various operating modes is selected so that there is no need to correct the fuel amounts in order to achieve an advantageous or required lambda value. According to the invention, the switch between various operating modes is adjusted so that a constant or stable combustion in the engine is maintained during the entire switch between the prevailing operating modes. Furthermore, with the invention, a switch between the various operating modes is accomplished without any noticeable torque changes in the engine. This is advantageous with respect to the comfort of the passengers of the vehicle. Also, the emissions of the engine can be reduced.
According to the invention, the lambda values corresponding to a required air/fuel mixture to the engine are calculated continuously for any of the possible modes of operation and for the requested mode of operation for the engine, both before and after a switch. In this connection, the term xe2x80x9cNOx adsorbing exhaust catalystxe2x80x9d used hereinafter refers to an integrated component having NOx adsorbing material, as well as material that functions as a conventional three-way catalyst. Further, the term xe2x80x9cmode of operationxe2x80x9d or xe2x80x9coperating modexe2x80x9d, refers to operation of a direct injected combustion engine according to a predetermined amount or length of time for injecting fuel and igniting an air/fuel mixture. For example, an operating mode of a Dl engine includes stratified and homogeneous modes of operation.
In this connection, the term xe2x80x9cfuel controlxe2x80x9d can refer to a nominal fuel control, wherein air and fuel are dosed into the engine in predetermined amounts during the prevailing driving condition (e.g., depending on the engine speed and the required torque). The term xe2x80x9cfuel controlxe2x80x9d can also refer to an air-based fuel control, wherein air is dosed into the engine according to a predetermined amount during prevailing driving condition (e.g., depending on engine speed and the requested torque) and fuel is dosed into the engine according to the amount of inflowing air.
In one embodiment of the invention, the switch from one operating mode to another is carried out at a lambda value that corresponds to a required fuel amount and an amount of inflowing air to the engine reaching a predetermined limit value.
This embodiment can include stratified and homogenous operating modes. Further, the limit value selected can correspond to one that provides a stable combustion in the engine without any substantial torque changes during the switch.
The embodiment can further include initiating the switch between nominal and air-based fuel control when the lambda value reaches a second predetermined limit value. Preferably, this second limit value is selected so that substantially no additional torque from the engine is provided during the switch of fuel control. Further, both limit values are preferably separated from each other.
In another embodiment of the invention, the switch is initiated based upon a driver-generated changed in required torque from the engine.
In a further embodiment of the invention, the engine is arranged in connection with a NOx adsorbing exhaust catalyst found in the exhaust system connected to the engine. In this embodiment, a compulsory switch in operating modes is initiated when NOx regeneration of the catalyst is required.